In order to execute the junction of cables for the transmission of electrical energy at medium and high voltages (higher than 10 KV), the extremities to be joined are deprived of the protection and insulation sheaths, so as to expose the electrical conductors and to make it possible to join them by means of welding or by means of clamping terminals. The junction area is then coated with suitable materials, such as putties, self-amalgamating tapes and such like, and over the extremities of the cables there is fitted a cover sleeve in elastomeric material, which has the functions of protecting and insulating the junction.
Covering sleeves are also applied to tubes, bars and such like, to obtain local protection against corrosion or for other purposes.
In order to fit a covering sleeve over a junction of electrical cables, it has been proposed to arrange the sleeves over a tubular element of rigid plastic materials, capable of keeping it under conditions of elastic expansion.
The tubular element is fitted over the extremity of one cable before the junction is executed and, once the connection between the conductors has been made, it is removed by slipping it out of the sleeve, which in this way, the sleeve can contract elastically and tighten itself over the cables at the junction.
Different forms of embodiments have been proposed to facilitate the operation of extracting the tubular supporting element.
In the British patent No. 1.292.608 there is described a hollow nucleus for supporting an elastic sleeve, formed by a pipe of plastic material having a deep helical groove on its external surface. There is thus defined a strip wound like a helix with the adjacent turns interconnected by a thin cordon which forms a preferential line of breakage and allows the separation of the turns by pulling one extremity of the strip. In this way, the pipe is removed in the form of a continuous thin strip, allowing the sleeve to contract over the cables.
The U.S. Pat. No. 4,389,440 describes a hollow nucleus for supporting an elastic sleeve, formed by a tape in plastic material wound in a spiral, having thinned-out edges, constrained at certain points so as to form a substantially rigid pipe, that can collapse and be removed to allow the abovementioned sleeve to be fitted.
The U.S. Pat. No. 4,503,105 relates to a tubular nucleus formed by a plastic pipe whose wall is provided with internal ribs, axially directed and spaced in a circumferential direction, and is externally cut along a helical line, so as to form a continuous helically-wound strip, held together by the axial ribs.
The European patent application No. 0,291,203 describes a helical support obtained from a tubular casing having notches extending through the wall thereof and arranged along the helical line and spaced in a circumferential direction and staggered, so as to leave in between the notches themselves connection areas of resistance such as to allow the helical support to be undone manually, by pulling on an extremity. The incisions are made by means of a tool constituted by a toothed cutting wheel.
When the helical support is obtained from a tubular casing with a helical incision, accomplished with tools operating by the removal of chips, the width of the incisions themselves is at least equal to the thickness of the tool.
With tubular supporting elements of the type described in the abovementioned patents, provided with external helical incisions, deformations of the internal wall of the sleeve made of elastomeric material are inevitable, and this can lead to supporting drawbacks.
In fact, the fitting of the sleeve over the tubular supporting element is executed during the manufacturing stage, and thus the sleeve and the tubular supporting element remain engaged for a long period of time before the tubular supporting element is removed to execute the fitting of the sleeve over a junction of electrical cables.
It thus occurs that the sleeve, mounted onto the tubular supporting element under conditions of high elastic expansion, exerts on this a strong centripetal compression, which, in the presence of discontinuities, such as incisions of notches, in the wall of the tubular element itself, leads the sleeve material to extend inside the incisions or inside the notches, with consequent deformations of its internal wall.
It so happens that these deformations are not completely eliminated in the short time span between the removal of the tubular supporting element and the contraction of the sleeve round the electrical cables of a junction. In fact, the materials that are suitable for accomplishing elastic sleeves for junctions of electrical cables, after remaining in conditions of elastic deformation for a certain period of time, exhibit a certain value of residual deformation which, at room temperature, can only be recovered after a few hours or days.
Thus, between the internal wall of the sleeve, which remains deformed, and the external surface of the insulation of the cables, inclusions of air are formed, with highly detrimental consequences, since, when air ionises in the presence of the strong electrical fields existing in the junction during use, can produce partial discharges, of an intensity equal to several tens or hundreds of picoCoulombs, which jeopardise the functionality of the sleeve and of the junction as a whole.
In the Italian patent application No. MI 91 A 001416, filed on 23 May 1991 in the name of the same Applicant, there is described a tubular supporting element of a sleeve made of elastomeric material, having a smooth and continuous external wall and provided with a helical groove, with a substantially triangular or trapeze-shaped cross-section, obtained on its internal wall.
This solution, which allows the external surface of the tubular support to be smooth, reduces the resistant cross-section of the tubular casing itself, with respect to the condition of the integral pipe, limiting the pressure that it can sustain for a given wall thickness.